Phosphor devices are used in lighting apparatus wherein the phosphor is remote from the exciting light source, i.e. there is no direct contact between the phosphor and the exciting light source. Therefore, they are also called remote phosphor devices. Remote phosphor devices can be used in various lighting applications, e.g. in RGB projection equipment, generating red (R), green (G) and blue (B) light for colored video projection. Other possible lighting applications comprise medical, architectural or entertainment lighting.
In prior art, remote phosphor devices, such as phosphor wheels, the phosphor is coated on a carrier plate. The phosphor is excited by exciting light, e.g. visible blue laser light (450 nm), impinging on the phosphor. The exciting laser light is wavelength-converted by the phosphor to generate light with longer wavelengths (e.g. broad spectral distribution with a peak about 520 nm for green light). Due to the physical nature of the wavelength-conversion, part of the exciting light energy is converted to heat (Stokes Loss). The greater the frequency difference between exciting and wavelength-converted light, the higher the Stokes Loss and thus the higher the thermal impact on the phosphor. Further effects, such as light absorbance, heat the phosphor additionally. The turning of the phosphor wheel facilitates removal of the heat generated in the phosphor.
The wavelength-converted light from the phosphor is gathered by a transmitting member, e.g. an optical element such as a lens made of glass, arranged in front of the phosphor wheel. Due to the turning of the phosphor wheel, a certain distance between the phosphor wheel and the transmitting member has to be kept. However, the gap causes optical losses. Firstly, there are reflection losses (Fresnel losses), because of different refractive indices of air (gap) and glass (transmitting member), which are independent of the size of the gap. Reflection losses increase with increasing angle of incidence. Secondly, the rays of the wavelength-converted light outside the acceptance angle of the transmitting member, i.e. rays which miss the entrance face of the transmitting member, are lost. Acceptance losses increase with increasing gap between phosphor surface and entrance face of the transmitting member.